Historians usually stick to facts, but sometimes it’s fun to play the “what if” game with history. What if president-elect John F. Kennedy had been assassinated in December 1960? What if Robert Kennedy had lived through Sirhan Sirhan’s attempt on his life and became president in 1968? What if Gerald Ford had corrected a misstep in the 1976 presidential debate with Jimmy Carter and won a second term? Long-time political journalist and CBS News correspondent Jeff Greenfield imagines all of these scenarios in his new book “Then Everything Changed: Stunning Alternate Histories of American Politics.”